In a drug delivery device, often, a piston or bung within a cartridge that contains a drug is provided. The piston is displaced with respect to the cartridge by a piston rod for delivering a dose of the drug from the cartridge. In general, it is desirable that the delivered dose of the drug matches the dose which was previously set by a user, i.e. the dose accuracy of the device should be high.
A drug delivery device is described in document EP 1 923 083 A1, for example.